> I think technically it's 28kW -rated (there's a consumer-installed limit, w/o boilermaker license), but the circuits support more w/o 80% derating applied

A tankless water heater is not considered a continuous load so there’s no need to apply the 80% rule.

A 60A 2P breaker will have a trip curve that results in a thermal trip for just under 100% of rated current in around 2-3 hours. The fast acting part of the trip curve is magnetic, longer duration trips are thermal.

Here’s a link to a Square D breaker guide: https://ressupply.com/documents/square_d/QO_and_QOB_Circuit_...

The trip curve on page 25 of the pdf applies to Square D QO plug-in (residential breakers are usually plug-in, commercial are bolted on) 2-pole breakers rated 120/240V from 45A-60A. Find the 1 (times rated current) at the bottom and follow it up the chart until it intersects with the black area of the trip curve, that is approximately when the breaker will trip at 100% of its rated ampacity. Look at the left hand side to see the time in seconds that it will trip in.

It’s hard to see exactly where it intersects, but it’s somewhere between 7000-10000 seconds, or 2-3 hours.

So, you need to apply the 80% rule to continuous loads because breaker trip curves are adjusted so the thermal overload trips in 3 or fewer hours at 100% of rated ampacity. If you look at .8 times rated load, the line never intersects the trip curve.

Here’s a manual for an A.O. Smith tankless water heater:

> https://assets.hotwater.com/damroot/Original/1000/100306523....

On page 10, the 4 element, 7kW per element unit draws 58.33A per 60A breaker, 7000/240 = 29.167A, two elements a piece for 58.33A per 60A breaker.

It’s lot cheaper to wire up a 28kW electric heater if you have 480V three-phase, it’s only 28000/480/1.732 = 33.68A, all you need is a 35A 3P breaker, three #10s and a #10 ground.

240V single phase needs two 60A 2P breakers, four #6s and two #10 grounds, or if it was a single-point connection, one 125A 2P breaker, two #1/0s and a #6 ground.

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ProllyInfamous 2 hours ago | parent [-]

The 28kW limit is from the Boilermakers Union, not ours [IBEW] =P

As much as I hate AFCI breakers, I do love a well-designed "stupid" heat-response timeout that's in compliance with the NEC. You're correct that residential waterheaters are not "continuous loads" – had slipped my mind.

I used a tankless/instahot heater (and helped install a few hundred in the early 2010s) and am so much happier with my hybrid/heatpump tank-type (it is so much cheaper to operate, requiring a relatively minimal upkeep of: an annual drainage).

Plus: there are no "miminum flow" requirements/bullshit, which results in some tempermental dishwashing among the water-conscientious (sp?).

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quickthrowman 2 hours ago | parent [-]

> The 28kW limit is from the Boilermakers Union, not ours [IBEW] =P

Ahh gotcha, they must’ve pushed for some good ol trade protectionism after electric boilers came out and high-power tankless water heaters are within their wheelhouse or something like that. I wouldn’t consider it a pressure vessel but I don’t blame them for scooping up the work, lol. I’m not in the union myself, but I do manage IBEW electricians and know enough to be dangerous ;)

> As much as I hate AFCI breakers, I do love a well-designed "stupid" heat-response timeout that's in compliance with the NEC. You're correct that residential waterheaters are not "continuous loads" – had slipped my mind.

I believe electric tank style water heaters under a certain size are considered continuous loads, but tankless are not.

> I used a tankless/instahot heater (and helped install a few hundred in the early 2010s) and am so much happier with my hybrid/heatpump tank-type (it is so much cheaper to operate, requiring a relatively minimal upkeep of: an annual drainage). Plus: there are no "miminum flow" requirements/bullshit, which results in some tempermental dishwashing among the water-conscientious (sp?).

Heat pump water heaters seem amazing, 25% of the power usage of a resistive heater, and especially for $250!

I wasn’t aware of minimum flow requirements for tankless heaters, I suppose it’s necessary to prevent overheating/steam or something? I mostly see tankless water heaters as part of emergency eyewash station installations, most commercial buildings around here either use boiler water for domestic hot water heating or have point-of-use tank water heaters near sinks/bathrooms.

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ProllyInfamous 2 hours ago | parent [-]

>IBEW electricians and know enough to be dangerous

You definitely sound just like us =P

>minimum flow [for tankless]

Yes, my brother has a kitchen pretty far from his tankless and if you don't have a disrespectful (i.e. anti-environmentalist) flow going, it's going to get cold and then stay that way for quite a while. It is aggravating, even as an occassional guest in his house – the whole damn line has to heat back up, again!.

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phil21 an hour ago | parent [-]

> . It is aggravating, even as an occassional guest in his house – the whole damn line has to heat back up, again!.

For point sources located far away from the heater, you are supposed to install a return loop. Modern tankless have a tiny (1-3 gallon) superheated tank and recirculation pump designed specifically for this use-case.

You can pry my continuous water heaters from my cold dead hands. What is much more annoying is running out of hot water when you have a peak guest load in your house right before an evening event and everyone is taking showers at the same time after a day out.

Since I use very little hot water otherwise, it pencils out for the environment too! The few times a guest is in a far guestroom and needs to use a small point of use hot water source, the few extra gallons of use to wait for it to kick in is a rounding error.

The two tankless heaters I have installed in my place are by far the single best upgrade I did since buying the house. I often comment on how much better my quality of life is with them vs. before they were installed.

I would never use a water heater with a tank ever again unless forced to. Other than air conditioning it is basically one of the top luxuries I work to provide for myself. My wife can take a bath, 3 other guests can shower all at the same time along with two loads of laundry and a dishwasher cycle going. No worries and no waiting around for an hour for hot water to regenerate. Since it's designed for peak loads and only spins up the second unit on-demand, it's much better in terms of energy use than a boiler designed to support those types of loads it sees 2 or 3 times a year at most.

If I were re-designing my system today I might do a heat pump water heater in-line with a continuous water heater, and the continuous only fires up once the tank runs empty.

	▲	ProllyInfamous 10 minutes ago \| parent [-]
		In your personal usage preferance (tankless), the only reason you would consider using a tank again is if you wanted a reasonably-efficient backup generator/offgrid/standby (or the free dehumidification/energy savings). I've used both and from an environmentalist living in a humid subtrop. rainforest, the hybrid tank (heatpump) makes most sense. Thankfully, they also have heating elements (and can run both heat sources, simultaneously).